In the transformer coil art, a number of methods have been adopted for holding or anchoring the turns of an electrical coil, to resist movement when the turns are subjected to the flow of current and electromagnetic forces tending to move them out of position. A commonly practiced method is to bond the turns of the coil to the layer insulation by the use of solid resinous adhesive layers or resin solution impregnated layer insulation. This method has not been entirely satisfactory, since by filling the coil with such continuous adhesive layers or completely resin solution impregnated materials, it is rendered impervious to the penetration of oil, which is essential in providing high impulse strength in transformers.
Attempts to solve this problem have been made by Ford in U.S. Pat. No. 2,942,217 where fibers were applied to resinous, adhesive coated insulating paper, to form an oil permeable spongy mat, limiting lateral movement of contacting, tightly wound conductor layers in a transformer coil. Ford, while supplying good oil permeability to the coil, was not entirely satisfactory, since the fibrous spongy layer could allow some conductor shifting. To provide improved porous solidification of transformer coils, which will withstand large surges of power with resulting high mechanical stresses, Ford, in U.S. Pat. Nos. 3,237,136 and 3,246,271 used discontinuously patterned resin solution impregnated kraft paper as the restraint. This method however averages only about a 0.25 mil (0.006mm) to 1.5 mil (0.036mm) adhesive thickness build. This low range can be inadequate for complete wire to paper bonding, providing insufficient short circuit strength, and if large pattern coatings are overlapped oil permeability can suffer. This impregnation with resin solution saturates the paper fibers under the adhesive pattern. When the patterned paper is subject to a high humidity atmosphere, the paper surrounding the adhesive pattern can swell such that in some cases, the adhesive pattern forms a depression and is rendered ineffective to bond coils.
Other methods of coating paper have provided thicker builds of resin using dry powder application. Corbett, U.S. Pat. No. 3,503,778; and Williams, U.S. Pat. No. 3,549,403 coat paper with a dry resin powder. Corbett fuses the plastic particles applied in a dusting chamber and Williams hot roll compresses the plastic particles applied by an electrostatic coating apparatus. Papers using powders applied by the Corbett and Williams methods might bond well but would not be porous.
What is needed then, is a method of making a highly porous yet completely and uniformly bonded electrical coil, using layer insulation having a discontinuous, porous, resin adhesive coating thickness of at least 0.25 mil (0.06mm), and preferably 1 to 5 mils. The coating must be applied in a manner to allow oil permeation through a plurality of laminated layers after the resin adhesive is set.